1. Field of the Invention
This invention relates to the field of aeration and circulation systems for ponds, lakes, sounds, treatment basins and other bodies of water; and especially to the field of aeration and circulation systems for bodies of water which experience periods of thermal-density stratification and oxygen depletion.
2. Discussion
Deep dimictic lakes typically exhibit a well mixed surface layer (epilimnion), a mid-depth where temperature decreases rapidly with increasing depth (metalimnion), and a uniformly cold deep layer (hypolimnion). Oxygen is supplied to the surface of the body of water from the atmosphere. Oxygen is also produced by photosynthesis within the body of water. When light penetration is limited, oxygen production by photosynthesis only occurs in upper strata. In more shallow water bodies such as, for example, lakes, estuaries, sounds and treatment basins that exhibit very high oxygen demand over the bottom, a weak or intermittent thermal stratification can occur and can result in oxygen depletion in deeper strata. The deep strata oxygen consumption rate and oxygen demands at the sediment/water interface can exceed the oxygen replenishment rate from the atmosphere and photosynthetic production near the surface.
The loss of dissolved oxygen from waters at various depths can have serious water quality consequences including:                Loss of desirable habitat for fish and other aerobic aquatic organisms;        Accumulation of nutrients and anaerobic respiration products such as iron, manganese, hydrogen sulfide, phosphorus, ammonia and other constituents; and        Increased eutrophication and degradation of resource quality for recreation, habitat, and water supply.        
The thermal density stratification and oxygen depletion is characterized by an index: relative thermal resistance to mixing (RTRM). Intervention may be required to increase and enhance circulation and aeration to improve and maintain water quality.